Turning string errors into constant ones (bug 818972)

DO NOT MERGE: it requires the solitude counterpart!
Merge pull request #299 from davidbgk/818972-error-constants

Turning string errors into constant ones (bug 818972)
restrict bounce to mozpay urls (bug 920966)
Merge pull request #302 from wraithan/restrict_bounce

restrict bounce to mozpay urls (bug 920966)
Abstract full-screen errors and add timeout to persona login (bug 870432)